1. Field of the Invention
The present invention relates to an optical pickup apparatus performing a reading operation of a signal recorded in an optical disc or a recording operation of a signal into the optical disc.
2. Description of the Related Art
There have been widespread optical disc devices capable of a signal reading operation and signal recording operation by applying laser light emitted from an optical pickup apparatus to a signal recording layer of the optical disc.
The optical disc devices are widely available in general, which use optical discs called CD (Compact Disc) and DVD (Digital Versatile Disc). As laser light for performing the reading operation of a signal recorded in a CD standard optical disc, infrared light with a wavelength of 780 nm is used, and as laser light for performing the reading operation of a signal recorded in a DVD standard optical disc, red light with a wavelength of 650 nm is used.
Thickness of a protective layer provided on a top face of a signal recording layer of the CD standard optical disc is specified at 1.2 mm, and a numerical aperture of an objective lens used for the reading operation of a signal from this signal recording layer is specified at 0.45. Thickness of a protective layer provided on a top face of a signal recording layer of the DVD standard optical disc is specified at 0.6 mm and a numerical aperture of an objective lens used for the reading operation of a signal from this signal recording layer is specified at 0.6.
FIG. 6 is a schematic diagram illustrating an optical system making up an optical pickup apparatus performing a reproduction operation of a signal recorded in a signal recording layer L provided in an optical disc D in the DVD standard, and a configuration of the optical pickup apparatus will be described with reference to FIG. 6.
In FIG. 6, reference numeral 1 denotes a laser diode emitting a laser beam, which is red light with a wavelength of 650 nm, and reference numeral 2 denotes a diffraction grating that is provided at a position where the laser light emitted from the laser diode 1 enters, and that splits the laser beam to generate a main beam which is 0th order diffracted light and sub beams which are ±1st order diffracted lights. Reference numeral 3 denotes a half-wave plate that the laser light having passed through the diffraction grating 2 enters, and that adjusts a polarization direction of the laser light emitted from the laser diode 1 to an S-direction or P-direction of linear polarization light.
Reference numeral 4 is a half mirror that is provided at a position where the laser light having passed through the diffraction grating 2 and the half-wave plate 3 enters, and that has a control film formed thereon which reflects the laser light in a direction of the optical disc D and which allows return light reflected from the signal recording layer L of the optical disc D to pass therethrough. Reference numeral 5 denotes a collimating lens that is provided at a position where the laser light reflected by the half mirror 4 enters, and that converts the incident laser light into parallel light.
Reference numeral 6 is a raising mirror that the laser light having been converted into the parallel light by the collimating lens 5 is incident, and that reflects the laser light to change an optical axis in a perpendicular direction. Reference numeral 7 is a quarter-wave plate that is provided at a position where the laser light reflected by the raising mirror 6 enters, and that polarizes the laser light incident from the side of the raising mirror 6 from the linear polarization light into circular polarization light and polarizes the return light which is laser light incident from the opposite side, from the circular polarization light into the linear polarization light.
Reference numeral 8 is an objective lens that is provided at a position where the laser light having passed through the quarter-wave plate 7 is applied, and that generates a spot in a shape suitable for the reproduction operation performed by focusing the laser light on the signal recording layer L provided in the optical disc D. The laser light focused on the signal recording layer L provided in the optical disc D by the objective lens 8 is reflected by the signal recording layer L to enter the objective lens 8 from the side of the optical disc D as return light.
The return light entering the objective lens 8 passes through the objective lens 8, and then enters the quarter-wave plate 7 to be converted by the quarter-wave plate 7 from the circular polarization light into the linear polarization light. The return light having been polarized as above is reflected by the raising mirror 6, and then enters the collimating lens 5.
The return light entering the collimating lens 5 passes through the collimating lens 5 to enter the half mirror 4. The laser light and the return light are reversed with each other in a polarization direction of the linear polarization light by a reciprocal transmission operation, which is an operation that the laser light passes through the quarter-wave plate 7 onward and the return light passes therethrough backward, and therefore, the return light entering the half mirror 4 as above is not reflected by the control film provided for the half mirror 4 but passes through the control film.
The half mirror 4 adds astigmatism to the return light passing therethrough in order to generate a focus error signal for a focusing control operation, however, it has a problem that coma aberration is generated due to its characteristics. Reference numeral 9 denotes an AS (Astigmatism) plate that provided at a position where the return light having passed through the half mirror 4 enters, and that is made up so as to enlarge the astigmatism generated in the half mirror 4 to become in a size suitable for generating a focus error signal and so as to correct the coma aberration generated in the half mirror 4.
Reference numeral 10 is a photodetector that is provided at a position where the return light having passed through the AS plate 9 is applied and that is made up so as to generate a focus error signal and a tracking error signal by using a change in spot shape formed by irradiation.
The optical system of the optical pickup apparatus to be used in general is configured as described above, and a generation operation of the tracking error signal will be described below referring to FIG. 2.
For a light receiving portion of the photodetector 10, there are provided a four-divided sensor portion 10a to which a main beam M in the return light is applied as shown in FIG. 2 and two-divided sensor portions 10b and 10c to which sub beams S1 and S2 are applied, respectively, are provided. The four-divided sensor portion 10a is made up of sensors A, B, C, and D as shown in the figure, while the two-divided sensor portions 10b and 10c are made up of sensors E, F and sensors G, H, respectively.
In such configuration, if a spot position of the laser light with respect to a signal track provided for the optical disc D is displaced in a radial direction of the optical disc D, that is, if tracking deviation occurs, a position of the main beam M formed on the four-divided sensor portion 10a by irradiation and positions of the sub beams S1 and S2 formed on the two-divided sensor portions 10b, 10c by irradiation, are displaced in a direction of an arrow A or B. As a result of this, an amount of light received by each of the sensors is changed.
A circuit diagram illustrated in FIG. 2 is for a tracking control operation called differential push-pull method. In FIG. 2, reference numeral 11 is a first adder for adding a signal obtained from the sensor A irradiated with the main beam M to a signal obtained from the sensor D irradiated therewith, reference numeral 12 is a second adder for adding a signal obtained from the sensor B irradiated therewith to a signal obtained from the sensor C irradiated therewith, reference numeral 13 denotes a first subtracter for subtracting an output signal obtained from the second adder 12 from an output signal of the first adder 11, reference numeral 14 denotes a second subtracter for subtracting a signal obtained from the sensor F irradiated with the sub beam S1 from a signal obtained from the sensor E irradiated therewith, and reference numeral 15 denotes a third subtracter for subtracting a signal obtained from the sensor H irradiated with the sub beam S2 from a signal obtained from the sensor G irradiated therewith.
Reference numeral 16 denotes a third adder for adding an output signal of the second subtracter 14 to an output signal of the third subtracter 15, reference numeral 17 denotes an amplification circuit for amplifying an output signal of the third adder 16 by K times (K is set based on a light amount ratio between a light amount of the main beam and a light amount of the sub beams) to be output, reference numeral 18 denotes a fourth subtracter for subtracting an output signal of the amplification circuit 17 from an output signal of the first subtracter 13, and its output signal is output to an output terminal 19 as a tracking error signal.
Supposing that signals obtained from each of the sensors A, B, C, D, E, F, G, and H are A, B, C, D, E, F, G, and H and the tracking error signal is TE, the tracking error signal TE is calculated by TE=(A+D)−(B+C)−K{(E−F)+(G−H)}, and such tracking error signal TE can be obtained from a circuit shown in FIG. 2. An art relating to an optical pickup apparatus performing the tracking control operation by such differential push-pull method is described in Japanese Laid-Open Patent Publication No. H08-339556.
a, b, and c of FIG. 5 show states where the main beam M in the return light is applied to the four-divided sensor portion 10a, in which the a and c show states where the spot position of the laser light is deviated with respect to the signal track, and the b shows a state where the spot of the laser light is positioned on the signal track.
As is obvious from such figures, the spot shape of the main beam M formed by irradiation on the four-divided sensor portion 10a is in a circular shape close to a perfect circle, since the coma aberration generated at the half mirror 4 is corrected by the AS plate 9. If the spot shape of the main beam M is in the circular shaped beam as mentioned above, a well-balanced push-pull signal can be obtained, and therefore, the tracking control operation can be performed accurately.
In the optical pickup apparatus shown in FIG. 6, there can be considered elimination of the AS plate 9 for cost reduction. If the AS plate 9 is eliminated, an astigmatism enlargement operation can not be performed, and therefore, the focus error signal generation operation is affected. However, it was confirmed that the focusing control operation can be carried out without trouble by devising a detection circuit.
On the other hand, if the AS plate 9 is eliminated, the coma aberration generated at the half mirror 4 can not be corrected, and therefore, the spot shape of the main beam M generated by irradiation on the four-divided sensor portion 10a becomes a deformed circular shape as shown in FIG. 4. If such deformed circular spot shape of the main beam is formed by irradiation on the four-divided sensor portion 10a, the push-pull signal which is a signal output in the track direction becomes unbalanced, and therefore, an tracking error signal generation operation can not accurately be performed, which causes a problem that the tracking control operation can not accurately be carried out, so that costs could not be reduced.